In an optical disc recording system, the equalization of intensity distribution (hereinafter termed xe2x80x9cbeam shapingxe2x80x9d) of a laser beam that is output from a semiconductor laser source is desired in order to optimize the shape of a light spot on a disc. For example, beam shaping prisms may be used to either compress or expand the diameter of an incident beam in a specific direction by refracting the beam. Specific examples of such beam shaping prisms are disclosed in Japanese Laid Open Applications H10-268112 and H10-62611. All of these beam shaping prisms are designed so that they not only perform beam shaping, but also can compensate for recording performance degradation caused by wavelength fluctuations of a semiconductor laser that is used to read the optical disc.
Wavelength fluctuations of a semiconductor laser frequently occur at the start of operation of a laser or when the luminous power of the laser is changed. Such wavelength fluctuations are said to be an unavoidable phenomenon in an optical disc system for recording signals on a disc medium when turning the laser ON/OFF. However, in the case where the output of a laser fluctuates from a reference wavelength, the output beam from a beam shaping prism will be emitted at a slightly different inclination as compared with the output beam at the reference wavelength, due to the index of refraction of the beam shaping prism being dependent on wavelength. For a recording-type optical disc system, this becomes a problem in that the position of a reading light spot that is formed on a disc will vary with a change in wavelength. This will cause signal degradation in recording onto a disc as well as in reading from a disc using a photo head.
In the above conventional examples, this problem is solved by using two prisms which are formed of materials that have different, refractive indexes and different dispersions. Thus, by using a pair of prisms that form an achromatic beam shaping system, the angle of an output beam is kept roughly constant so as to prevent a positional fluctuation of a light spot on a disc even when the wavelength of the incident beam fluctuates. Further, a beam shaping optical system that uses two cylindrical lenses in lieu of a pair of prisms has also been known.
An optical system is disclosed in U.S. Pat. No. 5,101,458 which enables the output direction of a beam emitted from a prism to be made substantially constant when the wavelength of an input light beam and the incident angle thereof vary. The technique disclosed in this patent leads light that is output from a waveguide into a prism arranged opposite to the waveguide. The output direction of this light is made constant for a change in wavelength by the action of a diffraction grating formed on the output face of the prism. However, the invention described in the above prior art patent employs both a waveguide and a prism, wherein the prism requires a change of the angle of incidence of the input beam to the prism. Therefore the basic idea of that invention is different from that of the present invention. In the present invention the angle of incidence of the input beam is constant and no waveguide path is needed as a necessary component.
The object of the present invention is to provide an achromatic beam shaping prism in which the output angle of an emitted beam remains substantially the same even when the wavelength of the incident beam changes and which uses a construction that enables both miniaturization and low cost.